1. Field of the Invention
This invention relates to the removal of potentially hazardous metals and/or trace elements from coal, and more particularly to a process for chemical cleaning of coal to remove mercury, either alone or along with other trace elements, prior to combustion of the coal, thereby reducing potential hazardous air pollutants (HAPs) in the combustion products.
2. Description of the Prior Art
It is common practice to clean or wash mined coal to remove debris, ash producing clays, and the like before the coal is burned. This cleaning process may involve a froth flotation, gravity separation, or other physical washing or cleaning process.
It is also known that coal washing processes are effective in removing a portion of the trace element content of coal, which trace elements remain in the coal cleaning waste streams. As a result, the quality of ground and surface water resources may be adversely affected if the trace elements discharged to impoundments or land fills exist in minerals that are or may become unstable. It is generally considered that these trace elements which are found in coal are relatively stable over geologic time periods, and that physical coal washing does not alter these minerals. This indicates that with the application of proper disposal techniques, trace elements removed by physical washing of coal should not materially contaminate ground or surface water. At the same time, some concern remains over the possibility and extent of contamination of ground and surface water, particularly if increased cleaning efficiencies result in a greater amount of the trace elements being released in the coal cleaning waste streams. It is therefore desirable to remove potentially hazardous trace elements by processes which permit their recapture for use, or for hazardous material disposal.
Of greater concern is that the release of HAPs to the atmosphere from burning coal containing HAP precursors such as mercury, arsenic, chromium and selenium, and mercury has been identified for potential regulation under the 1990 Amendments to the Clean Air Act. While some portion of all these elements may be removed by conventional coal cleaning processes, a major portion normally remains in the cleaned coal. For example, typical mercury reductions may be about 37% and selenium reductions may be about 40% by known commercial washing processes, leaving potentially hazardous amounts of these elements in the cleaned coal.
It is known that trace elements occur in coal in a wide variety of mineral and organic associations, and these associations vary from element to element and from coal to coal. The occurrence of mercury in coals is such that it is either associated with pyrite, embedded in the pyrite phase, finely dispersed in the pyrite, or is organically bonded and finely dispersed in the coal matrix. Of significance is the fact that mercury and its compounds formed during removal from coal using known chemical removal solvents were not stable. During treatment, once equilibrium is reached at some definite condition, the rate of removal from the coal into the solvent, as well as from the solvent to the vapor stage, equals the rate at which the mercury in the solvent solution returns to the coal particles and from the vapor phase back into the solvent. Thus, no noticeable increase in efficiency is obtained over an infinite length of time in which the coal is in contact with the solvent solution. In other words, the removal rate for mercury from the coal at equilibrium is the same as the return of mercury to the coal, and extended exposure does not increase the efficiency of the process.
It is, therefore, a primary object of the present invention to provide an improved method for removal of mercury from coal, utilizing a solvent extractant process. It is another object of the present invention to provide an improved method of removing mercury and other potential HAP precursor trace elements from coal.
Another object is to provide an improved method of removing potential HAP precursor trace elements from coal which has been previously cleaned by physical cleaning processes.
Another object is to provide a chemical leaching method of removing mercury from cleaned or uncleaned coal.
Another object is to provide an improved chemical method for removal of mercury, selenium, chromium and arsenic from coal using an ionizing and leaching solution which forms complex ions with at least one of the trace elements to be removed or with a host mineral in which at least one trace element resides.
Another object is to provide an improved chemical process for the removal of mercury, or mercury and other HAPs from coal using a solvent extractant capable of forming ions or radicals capable of reacting with constituents of pyrite and/or mercury, and/or to attack the bonds between pyrite and coal.
Another object is to provide an improved method of removing mercury from coal and for recovering the mercury so removed.
Another object is to provide an improved and economical process for cleaning coal to remove trace elements and controlling HAPs emissions, particularly mercury emissions, to prevent their discharge into the atmosphere upon burning of coal.